Heat exchangers have long been used as evaporators and condensers in heating, ventilating, air conditioning and refrigeration (HVACR) applications. Historically, these heat exchangers have been round tube and plate fin (RTPF) heat exchangers. However, all aluminum flattened tube and fin heat exchangers are finding increasingly wider use in industry, including the HVACR industry, due to their compactness, thermal-hydraulic performance, structural rigidity, lower weight and reduced refrigerant charge, in comparison to conventional RTPF heat exchangers.
A typical flattened tube and fin heat exchanger includes a first manifold, a second manifold, and a single tube bank formed of a plurality of longitudinally extending flattened heat exchange tubes disposed in spaced parallel relationship and extending between the first manifold and the second manifold. The first manifold, second manifold and tube bank assembly is commonly referred to in the heat exchanger art as a slab. Additionally, a plurality of fins are disposed between each neighboring pair of heat exchange tubes for increasing heat transfer between a fluid, commonly air in HVACR applications, flowing over the outer surface of the flattened tubes and along the fin surfaces and a fluid, commonly refrigerant in HVACR applications, flowing inside the flattened tubes. Such single tube bank heat exchangers, also known as single slab heat exchangers, have a pure cross-flow configuration. In an embodiment of flattened tube commonly used in HVACR applications, the interior of the flattened tube is subdivided into a plurality of parallel flow channels. Such flattened tubes are commonly referred to in the art as multichannel tubes, mini-channel tubes or micro-channel tubes.
Double bank flattened tube and fin heat exchangers are also known in the art. Conventional double bank flattened tube and fin heat exchangers, also referred to in the heat exchanger art as double slab heat exchangers, are typically formed of two conventional fin and tube slabs, one disposed behind the other, with fluid communication between the manifolds accomplished through external piping. However, to connect the two slabs in fluid flow communication in other than a parallel cross-flow arrangement requires complex external piping. For example, U.S. Pat. No. 6,964,296 shows a flattened tube and fin heat exchanger in both a single slab and a double slab embodiment with horizontal tube runs and vertically extending fins. U.S. Patent Application Publication No. US 2009/0025914 A1 shows a double slab flatted tube and fin heat exchanger wherein each slab has vertical tube runs extending between a pair of horizontally extending manifolds and includes corrugated fins disposed between adjacent tubes.
A concern associated with the use of flattened tube heat exchangers as condensers in HVACR applications is poor drainage of retained water from the external surface of the flattened tubes and fin matrix. The retention of water can be particularly problematic in flattened tube heat exchangers having horizontal tubes with high fin density, sufficient flattened tube depth and close flattened tube spacing common in condenser applications. In such constructions, water tends to collect on the flat horizontal surfaces of the heat exchange tubes in the spaces between the densely packed fins. The water collecting on the external surfaces of the heat exchanger tubes acts as an electrolyte and tends to accelerate corrosion and pitting of the tube surface. Water retention on the horizontal surface of the heat exchanger tube may also result in increased airside pressure drop and reduced air flow which adversely affects the thermal performance of the system. Any water collecting on the horizontal tube surface also constitutes a layer of added thermal resistance to heat transfer on the airside of the heat exchange tubes.
Accordingly, the need exists for a flattened tube finned heat exchanger that is substantially free draining of retained water off the horizontal flat surface of the flattened horizontally extending heat exchange tubes. The desire also exists for a flattened tube finned heat exchanger that is substantially free draining of water, while also achieving enhanced thermal performance. The need also exists for a double slab flattened tube finned heat exchanger of simplified construction and a method for assembling the heat exchanger for high volume semi-automated production.